Lasers produce an intense, coherent beam of light which is useful in many applications, including, for instance, medical surgery, fiberoptic communications, weapons, industrial heating and cutting, and radar. The term laser is an acronym for Light Amplification by Stimulated Mission of Radiation. Although the term laser is specific to electromagnetic radiation in the light frequency spectrum, apparatus are known for amplifying many frequencies of electromagnetic radiation by the stimulated emission of radiation. For instance, the term maser is an acronym for Microwave Amplification by Stimulated Emission of Radiation. The terms "laser" and "maser" are used interchangeably herein and are not intended to be frequency specific but to denote any device for amplifying electromagnetic energy by stimulated emission of radiation.
In present masers/lasers, an input electromagnetic wave can be amplified because, under proper conditions, an electron which is struck by a photon emits a photon coherent with the striking photon. The electron does not absorb the striking photon and thus the population of photons increases. More and more photons are produced until the system reaches saturation, at which point the number of photons, and thus the electromagnetic output power, cannot further increase.
Atomic lasers are the most common type of lasers. They amplify radiation at well-defined wavelengths which correspond to discrete energy transitions which may occur within the atoms which comprise the lasing medium of such lasers. For instance, an argon atomic laser produces light only of a distinct wavelength corresponding to the energy transitions which occur within argon atoms. In atomic lasers, different frequencies of electromagnetic radiation can be generated by using different lasing media.
The present invention is not an atomic laser/maser but rather belongs to another class of lasers/masers which depend on the interaction of electrons traveling in free space. Since such electrons are not tied to an atom, they are not limited to specific energy transitions within an atom. Such lasers/masers can produce radiation, over a broad range of frequencies, and in theory, throughout the entire electromagnetic spectrum, by proper control of system operating parameters. In practice, these devices are particularly adaptable for amplifying radiation in the microwave and millimeter ranges (i.e., as masers), which includes wavelengths from about 1 decameter to about 1 millimeter.
A double stream amplifier is one such radiation amplification device that operates based on the interaction of free electrons. Double stream amplifiers depend on mutual electron bunching of two beams to produce electromagnetic radiation of useful power. The power that can be emitted from a stream of electrons is normally only the power of each electron multiplied by the number of electrons. However, Maxwell's equations dictate that if electrons are "bunched" together in a group much smaller than the wavelength of the emitted radiation, the power of the radiation is given by the power of each electron in the bunch multiplied by the square of the number of electrons in the bunch. Bunching is necessary to cause stimulated emission of radiation of a useful amount.
A double stream amplifier amplifies electromagnetic radiation through the space-charge electric field interaction of two distinct streams of electrons traveling in vacuo in the same space at slightly different velocities parallel to one another. (The double stream amplifier was first introduced in 1949 by Andrew Haeff, Haeff, Andrew V., The Electron-Wave Tube--A Novel Method of Generation and Amplification of Microwave Energy, Proceedings of the I.R.E., January 1949.) In a double stream amplifier, two electron guns inject two beams of electrons parallel to one another into an axial magnetic guide field within an electron drift tube. The beams can have low velocities as in the experiments of Haeff, or they can have relativistic velocities (more than about 10% of the speed of light) with one beam traveling slightly slower than the other as in calculations of G. Bekefi and K. D. Jacobs, J. Appl. Physics, 53, 4113 (1982). As predicted by the statistical laws of entropy, the electrons comprising the high energy (higher velocity) beam tend to lose energy to the electrons of the low energy (lower velocity) beam, which tend to gain energy. This interchange of energy occurs through the interaction of the space charge electric fields associated with the faster and slower moving electrons. The result is that, on the average, the fast electrons slow down and the slow ones speed up.
As explained more fully in the above-referenced articles, the interaction of the two beams and a disturbance (an electromagnetic wave which may be injected or may be an ambient background "noise" wave) causes space charge density, i.e., electron bunching, and stimulated emission of radiation by the electrons.
FIG. 1 illustrates a typical double stream amplifier 10 of the prior art. Circuitry 12 is provided to drive first and second cathodes 14 and 16 to inject first and second electron beams 18 and 20 into an electron drift tube 22. The circuitry 12 for driving the cathodes is configured such that the electron beams 18 and 20 can (but need not) have relativistic speeds (i.e., at least about 10% the speed of light) which are slightly different from each other.
Although the beams are shown distinctly as 18 and 20 in the figure, it should be understood that the electrons of the beams interact in the tube and are not physically separated by a distance more than a wavelength of the radiation. A solenoid 26 is electrically charged to provide an axial magnetic field in the drift tube parallel to the longitudinal axis of the drift tube, and thus parallel to the direction of propagation of the electrons. An RF source (for example, a magnetron 28) generates an electromagnetic wave of the interaction frequency of the system which is directed from the magnetron into the drift tube through a configuration of waveguides 30. The interaction frequency is a function of various parameters of the system which may be adjusted to provide the desired frequency. The interaction frequency is given by: EQU .omega..sub.int .apprxeq.2(.beta..sub..parallel..sup.2 .gamma..sub.81 )(.gamma..sub..parallel. /.DELTA..gamma..sub..parallel.).omega..sub.p
where,
.beta..sub..parallel. =average parallel velocity of electrons normalized by the speed of light, i.e., v.sub.par /c,
.gamma..sub..parallel. =average parallel kinetic energy parameter of the electrons in the tube,
.DELTA..gamma..sub..parallel. =difference in the parallel kinetic energies of the two beams, and ##EQU1## where, N=number of electrons per cubic meter,
e=charge of an electron=1.602.times.10.sup.-19 C, PA0 m=mass of an electron=9.1.times.10.sup.-31 kg PA0 .epsilon..sub.o =permitivity of free space=8.85.times.10.sup.-12 farad/meter PA0 .gamma.=average total energy parameter of the electrons.
Amplification of the magnetron wave at the interaction frequency occurs due to the mutual interaction of charge density perturbations (electron bunching) of the two co-propagating electron beams traveling at different speeds.
The efficiency of conventional double stream amplifiers is fairly low, typically on the order of less than 10%. This is because the large gain in such amplifiers causes rapid electron thermalization, which tends to destroy the two-beam nature of the system. Efficiency is the percentage of the power in the electron beam which is converted into emitted radiation power. The power in the electron beam is simply the voltage times the current.
Another drawback of the double stream amplifier is that its output frequency fluctuates. In a double stream amplifier, the radiation frequency is proportional to the square root of the electron number density (i.e., the square root of the current), which is not readily controlled under present day electron beam generator technology. The fluctuations in the space charge density (i.e., current) result in poor frequency stability and the amplified signal tends to be noisy.
In another type of laser/maser, known as a free electron laser/maser, stimulated emission of radiation is induced by the interaction of (1) a single stream of free traveling electrons, (2) an undulating magnetic field (typically produced by a magnetic wiggler) which causes the electrons to gyrate transverse to their direction of propagation, and (3) an electromagnetic wave traveling in the same direction as the electrons.
In a free electron laser/maser, an electron beam is injected in vacuo into an electron drift tube often having an axial magnetic guide field parallel to the direction of propagation of the electrons. The guide field holds the electron beam together because, in the absence of the field, the electrons would disperse due to their natural tendency to repel one another.
FIG. 2 shows a typical prior art free electron maser. An electron gun 31 injects an electron beam 32 into an electron drift tube 33. A solenoid 34 produces a uniform axial magnetic field in the drift tube parallel to the direction of propagation of electrons. A magnetic wiggler 35 is driven with current to produce an undulating transverse magnetic force in the full length of the drift tube 33. The undulating magnetic field is selected to impart a transverse rotation to the electrons. A magnetron or other RF source 36 and waveguide configuration 37 inject an electromagnetic wave into the drift tube at 38. The wave injected by the magnetron should be of the desired output frequency of the maser.
When the electrons enter the wiggler magnetic field, their propagation changes from a straight line propagation to a helical propagation as they continue traveling parallel to the longitudinal axis of the tube. The wiggler field and electron beam velocity should be selected in accordance with the equation below such that the interaction frequency of the system corresponds to the desired output frequency (which should also be equal to the magnetron frequency); EQU .omega..sub.int .apprxeq.(1+.beta..sub..parallel.).gamma..sub..parallel..sup.2 (mk.sub..omega. .beta..sub..parallel. c)
where,
.beta..sub..parallel. =average parallel velocity of electrons normalized by the speed of light, i.e., v.sub..parallel. /c,
.gamma..sub..parallel. =(1-.beta..sub..parallel.).sup.2-1/2 =average parallel kinetic energy parameter of an electron in the tube
m=1, 2, 3 . . . and
k.sub..omega. =2.pi./1.sub..omega., where 1.sub..omega. is the periodicity of the wiggler field.
The gyrating relativistic electrons interact with the injected radiation to cause electron bunching and stimulated emission of radiation so as to amplify the magnetron input wave.
As is also true of double stream lasers, it is not necessary to inject a wave. In the absence of an injected wave, the laser would amplify an ambient background noise wave at or near the interaction frequency.
The operation of free electron lasers/masers is well known in the related art and various references are available should a more complete description of these masers be desired. For instance, the Encyclopedia of Laser and Optical Technology, Robert A. Meyers, Editor, Harcourt Brace Jovanovich, 1991, provides a detailed description of free electron masers as well as citations to additional references with more detailed descriptions.
A drawback of free electron lasers/masers is that they require large accelerators to produce electron beams of the required velocity and long wigglers to achieve sufficient gain. These requirements become particularly troublesome as the desired output frequency increases. These requirements limit the practical application of single stream free electron masers to a few laboratories.
A single beam electron cyclotron maser (CARM), yet another prior art system, operates on a different principle. The electrons undergo helical rotation in a uniform axial magnetic field after having acquired rotational motion by means of a short helical wiggler or kicker.
FIG. 3 shows a typical single beam electron cyclotron maser. The output frequency is given by EQU .omega..sub.int =(1+.beta..sub..parallel.).gamma..sub..parallel..sup.2 m.OMEGA.
where .OMEGA.=e.beta..sub..parallel. /m.gamma.=the electron cyclotron frequency associated with the axial magnetic field.
W. Chunyi and S. Liu, Double-Stream Electron Cyclotron Maser, Int. J. Electronics, 1984, Vol. 57, No. 6, 1191-1204 disclose a double stream electron cyclotron maser operating in fast TM.sub.o,1 or TE.sub..sub.o,1 electromagnetic empty waveguide modes. For interaction to occur in these modes, high axial magnetic fields and/or high beam energies, as well as positive cyclotron mode numbers on both beams are necessary. Theoretically this achieves high gain and broad band operation but no significant upshift in operating frequency.
Accordingly, it is an object of the present invention to provide a new high frequency double stream cyclotron laser/maser.
It is a further object of the present invention to provide a new double stream cyclotron laser/maser requiring much lower magnetic guide field strengths and beam energies compared to single stream free electron lasers/masers.
It is another object of the present invention to provide a double stream cyclotron laser/maser which can be energized by a table-top size particle accelerator.
It is yet another object of the present invention to provide a less noisy double stream laser/maser.
It is one more object of the present invention to provide an improved double stream laser/maser which outputs an extremely frequency stable output signal.
It is a further object of the present invention to provide a laser/maser which can produce higher frequency radiation with lower beam energy requirements than prior art lasers/masers.
It is a further object of the present invention to couple a growing slow space-charge wave of double stream cyclotron interaction to a slow wave electromagnetic structure.
It is yet another object of the present invention to incorporate a growing space-charge wave of double stream cyclotron interaction into a klystron-type geometry.